Corrosion on Canadian Nickels

I had been storing my Canadian nickel collection in a generic "Nickels" Whitman folder for many years. Recently I purchased the official Canadian Nickel Whitman folders and began to transfer the coins from one folder to another. To my surprise, many of the nickels had green corrosion or oxidation marks on the edges and the rims. The marks were even present inside of the folder pockets. I have begun the process of removing such coins from my collection and replacing them with extras where possible, but in some cases the green coin was the only one I had.
I know there is nothing I can really do to remove this (nor would I want to, for fear of damaging the coin), but I have two questions:
1. What causes this? Were my coins stored in an area with too much moisture or humidity?
2. Will this condition spread a) to expand more on the coin and/or b) to other coins stored nearby?
I know there is nothing I can really do to remove this (nor would I want to, for fear of damaging the coin), but I have two questions:
1. What causes this? Were my coins stored in an area with too much moisture or humidity?
2. Will this condition spread a) to expand more on the coin and/or b) to other coins stored nearby?